Field of the Invention
The invention relates to a catalyst including titanium oxide, vanadium oxide and at least one further catalytically active component for reacting organic compounds present in a gas mixture.
For the purposes of the present invention, organic compounds are generally hydrocarbons, solvents, carbon monoxide, cyclic and aromatic compounds as well as halogenated hydrocarbons, in particular dioxins and furans. Such gas mixtures can be, for example, automobile exhaust gases, waste air from industry, exhaust gases of stationary combustion engines and flue gases from combustion plants, e.g. power stations and waste incineration plants.
German Published, Non-Prosecuted Patent Application DE 38 04 722 A1 discloses a process and a catalyst for eliminating organic compounds from a waste gas. With regard to the process, it is disclosed that the waste gas containing the organic compound is fed together with oxygen, in particular together with air, within a prescribed temperature range, for example from 250 to 500.degree. C., to a selective catalyst which converts the organic compound into environmentally compatible reaction products such as carbon dioxide and water vapor. The acid resistance is an advantage specifically in the case of dioxin/furan degradation (formation of HCl, HF). A catalyst which is preferred in that case is a catalyst including a mixture of oxides. The catalyst under consideration in that case includes a large proportion by weight of titanium dioxide, preferably in the anatase crystal form. Compounds and/or oxides and/or mixed oxides of the transition metals chromium, manganese, cobalt, nickel, copper, vanadium, zinc, molybdenum and tungsten, but preferably chromium oxide, are added to the titanium dioxide as components which are particularly catalytically active.
Furthermore, Published European Patent Application 0 290 947 A1 discloses a catalyst for the oxidation of ammonia (NH.sub.3). That catalyst too includes a considerable proportion by weight of up to 50% of titanium dioxide which should preferably be present in the anatase crystal modification. That is because the anatase modification usually has a higher BET surface area than the rutile modification. Such a catalyst has a high catalytic activity due to the high BET surface area. German Published, Non-Prosecuted Patent Application DE 38 04 722 A1 and Published European Patent Application 0 290 947 A1 disclose a process for producing such a catalyst for the degradation of organic compounds. In that process, titanium dioxide is milled and intimately mixed with the active components and filtered. A binder is added to the resulting filter cake and the resulting mass is mixed and made into a predetermined shape, e.g. applied to plates or extruded to form honeycombs or pellets. The shaped composition is subsequently dried and subjected to a heat treatment or calcined.
The reaction that is carried out using such a catalyst is exothermic and for that reason the reaction can result in local overheating, known as hot spots, in the catalyst. Since the catalyst is formed predominantly of titanium dioxide of the anatase type, the specific surface area of the catalyst is irreversibly decreased by those hot spots because titanium dioxide of the anatase type is transformed into the rutile type with a change in the particle size. That results in the activity of the catalyst becoming worse with increasing operating time.
As an alternative to titanium dioxide as the base material for the catalyst, it is also known that a catalyst can be made up on the basis of aluminum oxide. Such a catalyst including aluminum oxide has little susceptibility to temperature stresses, in particular to hot spots. However, it has been found when using such a catalyst that, in contrast to titanium dioxide catalysts having the customary anatase modification, the catalyst does not have sufficient acid resistance, particularly in the degradation of halogenated hydrocarbons.
Furthermore, Published European Patent Application 0 264 747 A1 discloses a V.sub.2 O.sub.5 --TiO.sub.2 catalyst for reacting o-xylene in which the titanium oxide is present in the rutile form. In addition, German Published, Non-Prosecuted Patent Application 2 401 160 discloses a V.sub.2 O.sub.5 --TiO.sub.2 catalyst for the gas-phase oxidation of butene to give acetic acid, where more than 50% of the titanium dioxide is in the rutile form.